The evaluation and debugging of high-speed digital circuits and systems often requires measurement and display of signal waveforms. Often, these measurements are made using test equipment such as a logic analyzer, an oscilloscope, or a spectrum analyzer. A portion of a signal from the circuit or system under test (often referred to as the target) is provided to the test equipment via a probe.
There are important considerations when designing a probe. For example, it is desirable for the probe to provide an accurate representation of the target signal to the test equipment. Moreover, it is desirable for the probe not to impact the target signal significantly.
The accuracy of the probe measurement is impacted by a number of factors. One factor is the inductive loading of the probe. Because inductance increases with signal frequency, at high frequencies a probe can have a high impedance. While a high probe impedance has minimal impact on the signal traveling along a target transmission line (target signal) and thus is substantially non-intrusive, there may not be sufficient signal power provided from the target to the test equipment for reliable analysis.
Another factor that can influence the accuracy of the probe measurement is capacitive loading by the probe. The capacitive loading of the target signal by the probe can cause deterioration of the target signal. For example, capacitive loading can decrease the rise time, the fall time, and the bandwidth of the target signal. Thus, the measured signal may not be an accurate representation of the target signal.
There is a need for a signal probe and probe assembly that overcomes at least the shortcomings described above.